1. Field of the Invention
This invention relates to a fluorescent display device, and more particularly to an improvement in a fluorescent display device which has been conventionally used for household appliances such as an audio system, a VTR, an electronic oven and the like.
2. Description of the Prior Art
A fluorescent display device has been recently used in various fields as well as for household appliances. For example, it is often mounted on a vehicle for the purpose of being used for a speedmeter, a tachometer, an oil indicator and the like arranged on an instrument panel of the vehicle.
Display units used for meters mounted on a vehicle include a direct-reading type display unit wherein a luminous display of a fluorescent display device mounted on an instrument panel provided at a dashboard of the vehicle is directly observed and a so-called head up display unit as disclosed in Japanese Pat. Application Laying-Open Publication No. 182541/1982 and the like wherein a luminous display of a fluorescent display device arranged in a dashboard is reflected from a front glass of a vehicle to form an image of the display in a visual field in front of the front glass, so that a driver may observe an information without averting his eyes from the front glass.
The present invention is directed to a fluorescent display device of high luminance which is also used for the head up display unit.
The head up display unit, as described above, is so constructed that a luminous display of a fluorescent display device arranged in a dashboard is reflected from a front glass of a vehicle to form an image of the display in a visual field in front of the front glass. Accordingly, it is widely known that the head up display unit is deteriorated in visibility in the daytime wherein it is light outside, unless it exhibits high luminance as compared with the conventional direct-reading type display unit.
In order to avoid such a problem, a fluorescent display device to be used for the head up display unit is required to exhibit higher luminance.
A conventional fluorescent display device is generally constructed in such a manner that a wiring pattern and anode conductors each are formed of a conductive material on a glass substrate and phosphor layers are deposited on the anode conductors, resulting in formation of anodes. The fluorescent display device also includes mesh-like grids arranged above the anodes and filamentary cathodes stretched above the grids, so that electrons emitted from the filamentary cathodes are impinged on the phosphor layers while being accelerated and controlled by the mesh-like grids, to thereby cause the phosphor layers of the anodes to emit light. Accordingly, luminescence of the anodes is observed through the mesh-like grids and a transparent front cover. Unfortunately, such construction causes the mesh-like grids to reduce luminance of the anodes.
In order to minimize such a problem, a plane grid structure free of any mesh-like grid as shown in FIG. 4 was proposed for a fluorescent display device. More particularly, the plane grid structure includes a glass substrate 1 and wiring conductors 2 formed on the glass substrate 1 by depositing an Ag paste on the glass substrate by thick film printing. On the wiring conductors 2 is formed an insulating layer 4 provided with through-holes 3 by thick film printing. Also, the structure includes anode conductors 5 and plane grids 6 arranged on the insulating layer 4 in a manner to be electrically connected to the wiring conductors 2 via the through-holes 3. The anode conductors 5 and plane grids 6 are formed of a conductive graphite paste into the same height by thick film printing. Then, on the anode conductors 5 are deposited phosphor layers 7, which form anodes 8 in cooperation with the anode conductors 5. Above the so-formed anodes 8 are stretchedly arranged filamentary cathodes 10. Thus, the fluorescent display device of the plane grid type is constructed into a laminated structure by thick film printing. Accordingly, a failure in formation of a gap of 0.2mm or more between the anode conductors 5 and the plane grids 6 has a possibility of causing short-circuit to occur between the anode conductors 5 and plane grids 6 due to sagging of the paste used for forming the anode conductors and plane grids or depending on accuracy of the thick film printing. Also, in the conventional fluorescent display device of this type, when a width of each of the grids 6 required to insure accuracy of the printing and function of the grid is set to be at least 0.5mm and a gap between an end of the anode conductor 5 and the phosphor layer 7 is set to be 0.2mm, it is required to provide a gap S of 1.0mm or more between phosphor patterns of adjacent luminous segments. This causes the gap S between the segments to be enlarged to a degree sufficient to deteriorate continuity of figures displayed, resulting in failing to provide aesthetic luminous display.
Also, the grids each are positioned below a surface of the phosphor layer 7, so that it fails to satisfactorily exhibit its function. This often leads to a display defect phenomenon that luminous segments desired to emit light are partially fail to carry out emission due to a negative electric field of luminous segments adjacent thereto which are not desired to emit light.
In order to solve the above-described problem of the conventional fluorescent display device due to the thick film printing, a fluorescent display device which includes a metal film pattern as shown in FIG. 3 is proposed. More specifically, in the proposed fluorescent display device, wiring conductors 12, anode conductors 15 and plane grids 16 arranged on a glass substrate 1 are formed of a metal film and phosphor layers 7 are deposited on the anode conductors 15. It is disclosed in Japanese Patent Application Laying-Open Publication No. 59639/1985.
In the proposed fluorescent display device, the metal film pattern is formed by subjecting a metal film to etching, so that a gap between luminous segments may be narrowed. However, the grids are likewise positioned below the phosphor layers, to thereby still fail to eliminate such a display defect as described above due to a negative electric field of adjacent luminous segments which are not desired to emit light. Also, the fluorescent display device has another disadvantage that light emitted from each of the luminous segments is reflected from a surface of the metal film plane grid to cause a luminous display of the device to be doubled, resulting in failing to provide the display with an aesthetic property. Further, external light is reflected from the metal film, so that contrast between the display segments and their background is reduced to deteriorate visibility of the display.